System and method for supporting compatibility checking for lambda expression

ABSTRACT

A system and method can support compatibility checking in a programming language environment. The programming language environment can check whether a target type associated with an expression in the programming language environment is functional, wherein the target type is functional when it has exactly one method meeting one or more criteria that define that method as relevant in a counting system. Then, the programming language environment can derive a function descriptor from the target type, which is context-dependent. Furthermore, the programming language environment can type-check the expression with the function descriptor associated with the target type.

INCORPORATION BY REFERENCE

Each of the following applications are hereby incorporated by reference: application Ser. No. 15/402,319 filed on Jan. 10, 2017; application Ser. No. 13/779,536 filed on Feb. 27, 2013 and application No. 61/669,771 filed on Jul. 10, 2012. The Applicant hereby rescinds any disclaimer of claim scope in the parent applications or the prosecution history thereof and advises the USPTO that the claims in this application may be broader than any claim in the parent applications.

COPYRIGHT NOTICE

A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to the following patent application, which is hereby incorporated by reference in its entirety:

U.S. Patent Application Ser. No. 13/779,557, filed Feb. 27, 2013, now U.S. Pat. No. 9,195,442 issued Nov. 24, 2015, entitled “SYSTEM AND METHOD FOR COMPILING LAMBDA EXPRESSION IN A PROGRAMMING LANGUAGE ENVIRONMENT”.

FIELD OF INVENTION

The present invention is generally related to computer systems and software, and is particularly related to supporting compilation in a programming language environment.

BACKGROUND

An object-oriented programming language, e.g. the Java™ programming language, can dynamically encapsulate program behavior with methods in classes. However, the objects in the object-oriented programming language tend to be relatively heavyweight, due to the instantiation of separately-declared classes wrapping a handful of fields and different methods. Given the increasing relevance of callbacks and other functional-style idioms, it is beneficial that modeling code as data in the object-oriented programming language can be as lightweight as possible.

This is the general area that embodiments of the invention are intended to address.

SUMMARY

Described herein are systems and methods that can support compatibility checking in a programming language environment. The programming language environment can check whether a target type associated with an expression in the programming language environment is functional, wherein the target type is functional when it has exactly one method meeting one or more criteria that define that method as relevant in a counting system. Then, the programming language environment can derive a function descriptor from the target type, which is context-dependent. Furthermore, the programming language environment can type-check the expression with the function descriptor associated with the target type.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an illustration of supporting compatibility checking in a programming language environment in accordance with various embodiments of the invention.

FIG. 2 illustrates an exemplary flow chart for supporting compatibility checking in a programming language environment in accordance with an embodiment of the invention.

FIG. 3 shows an illustration of checking compatibility for a wildcard parameterized type in a programming language environment, in accordance with various embodiments of the invention.

DETAILED DESCRIPTION

The invention is illustrated, by way of example and not by way of limitation, in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” or “some” embodiment(s) in this disclosure are not necessarily to the same embodiment, and such references mean at least one.

The description of the invention as following uses the Java™ programming language environment as an example for a programming language environment. It will be apparent to those skilled in the art that other types of programming language environments can be used without limitation.

Described herein are systems and methods that can support compatibility checking in a programming language environment.

FIG. 1 shows an illustration of supporting compatibility checking in a programming language environment in accordance with various embodiments of the invention. As shown in FIG. 1, an expression, e.g. a lambda expression 101, can be supported in a programming language environment 100, e.g. using the Java™ programming language.

Lambda expressions 101 can be anonymous methods, which are aimed at addressing the “vertical problem,” or bulky syntax, by replacing the machinery of anonymous inner classes with a syntactically lighter-weight mechanism. The general syntax of the lambda expression 101 can include an argument list 111 and a body 112. Additionally, the syntax of the lambda expression 101 can include an arrow token “->.”

In the example of the Java™ programming language environment, the argument list 111 can include one or more parameters. The body 112 can be either a single expression or a statement block. In the expression form, the body is simply evaluated and returned. In the block form, the body is evaluated like a method body—a return statement returns control to the caller of the anonymous method. Furthermore, non-local control elements, e.g. break and continue, are illegal at the top level in the lambda expression, but are permitted within loops. If the body produces a result, every control path is required to return something or throw an exception.

The following are examples of lambda expressions.

-   -   (int x, int y)->x+y     -   ( )->42     -   (Siring s)->{System.out.println(s);}

The first lambda expression takes two integer arguments, named x and y, and returns x+y. The second lambda expression takes no arguments and returns the integer 42. The third lambda expression takes a string and prints it to the console, returning nothing.

The lightweight syntax for the lambda expression 101 can be optimized for the common usage in various programming language environments 100. In the example of the Java™ programming language environment, the expression-body form eliminates the need for a return keyword, which could otherwise represent a substantial syntactic overhead relative to the size of the expression.

A compiler 120 in the programming language compiling environment 100 can derive and/or infer various types associated with the lambda expression 101 that can be used for compatibility check. Such types can include one or more parameter types 121, one or more return types 122 from the expression body 112 and one or more exception types 123, or thrown types, from the expression body 112. For example, when the argument list 111 in the lambda expression 101 includes a typeof expression, then the compiler 120 can infer the parameter type 121 at runtime.

In accordance with various embodiments of the invention, the lambda expression 101 can have a type that can be deduced at compile time. When the lambda expression 101 appears in a programming context 110, it should be compatible with a type expected in that context, which is referred to as a target type 102. If the lambda expression 101 is not compatible with the target type 102, an error can occur at compile time.

In accordance with various embodiments of the invention, the functional interface 104, or class, can be used for imputing an isomorphism between the set of typed functions and a subset of legal nominal types in an object-oriented programming language. The functional interface 104, or class, which may be considered as “functions in disguise,” can have one method meeting one or more criteria that define that method as relevant in a counting system. In the example of the Java™ programming language environment, a functional interface 104 can have only one method excluding the methods that are inherited from the Object class. Additionally, each functional interface 104, or class, can have a function descriptor, which is the method type of the single method.

For example, in the Java™ programming language specification, the following interfaces can be considered as functional.

-   -   java.lang.Runnable with a function descriptor ( )->void     -   java.util.concurrent.Callable with a function descriptor ( )->V     -   java.security.PrivilegedAction with a function descriptor ( )->T     -   java.util.Comparator with a function descriptor (T,T)->int     -   java.io.FileFilter with a function descriptor (File)->boolean     -   java.nio.file.PathMatcher with a function descriptor         (Path)->boolean     -   java.lang.reflect.invocationHandler with a function descriptor         (Object, Method, object[])->object     -   java.beans.PropertyChangeListener with a function descriptor         (PropertyChangeEvent)->void     -   java.awt.event.ActionListener with a function descriptor         (ChangeEvent)->void     -   javax.swing.event.ChangeListener with a function descriptor         (Action Event)->void

The compiler 120 can structurally (i.e., without instruction from the user) recognize which interfaces/class has this characteristic or is functional, mechanically derive a function descriptor for the interfaces/class, and use that function descriptor for representing lambda expressions in a nominal object-oriented type system.

The lambda expression 101 may only appear in a programming context 110 that has a target type 102. The lambda expression 101 can be context-dependent, which means that the lambda expression 101 can be associated with a target type 102 that is expected to be different in different programming contexts 110.

In accordance with various embodiments of the invention, the compiler 120 can infer a target type 102 of the lambda expression 101 based on the programming contexts 110, since a target type 102 can be expected in the programming context 110, in which the lambda expression appears.

Furthermore, the compiler 120 can check whether the target type 102 is functional based on its structure. The compiler 120 can determine that the target type 102 is functional when the target type 102 represents a single function contract, e.g. implementing a functional interface 104 that has only one abstract method.

Additionally, the compiler 120 can use a process for identifying whether an interface is a functional interface 104 in the programming language environment 100. Such a process can be more complex than simply counting method declarations, since an interface may inherit methods from multiple parents that logically represent the same method, or it may redundantly declare a method that is automatically provided by the class Object, e.g. toString( ).

After determining that the target type 102 is functional, the compiler 120 can derive a function descriptor 103, i.e. method signatures, from the target type 102. The function descriptor 103 of a functional interface 104 can include a method type, type parameters, formal parameter types, return types, and thrown types.

Then, the compiler 120 can check whether the different types 121-123 associated with the lambda expression 101 are consistent with the function descriptor 103 for the target type 102.

In accordance with various embodiments of the invention, a lambda expression 101 can be considered as compatible with a target type 102, T, if the following conditions hold:

-   -   T is a functional interface type,     -   The parameters of the lambda expression are compatible with T,         e.g. the lambda expression has the same number of parameters as         T's method, and those parameters' types are the same or         assignment-compatible,     -   Each expression returned by the lambda body is compatible with         T's method's return type, and     -   Each exception thrown by the lambda body is allowed by T's         method's throws clause.

Thus, the programming language environment 100 allows programmers to safely express what they mean without excessive syntactic overhead.

For example, by taking advantage of the target typing, the programming language environment 100 allows the lambda expression 101 to avoid repeating parameters types 121 for the lambda expression 101, since the compiler 120 can have knowledge of the parameter types 121 used by the lambda expression 101 based on the target type 102.

For example, in the Java™ programming language environment, there is no need for specifying parameters types for variable s1 and s2, since the Java™ compiler can derive the String parameter types based on the target type, Comparator.

-   -   Comparator<String>c=(s1, s2)->s1.compareToIgnoreCase(s2);

In addition, when there is just one parameter, the type of which can normally be inferred, the parentheses surrounding a single parameter name can be optional.

-   -   FileFilter java=f->f.getName( ).endsWith(“.java”);     -   button.addActionListener(e->ui.dazzle(e.getModifiers( )));

Furthermore, different programming contexts 110, such as an assignment context, an invocation context, and a casting context, can be supported in the programming language compiling environment 100. Since a different target type 102 is expected to be associated with the lambda expression 101 in a different programming context 110, the type-checking of the lambda expression 101 in a different programming context 110 can be different.

From the perspective of a compiler 120, the lambda expression 101 can be used to create an instance of a target type 102. Also, the compatibility check between return expression of the lambda body and expected return types in the target type 102 can depend on the context.

The following example shows how the interpretation of the body of a lambda expression can vary depending on the context. In this example, a functional interface can be defined as

interface Target { int m( ); }

Assuming a variable, longVar, is defined with a long type, the following expression, which is in an assignment context, may be rejected by the compiler 120 for the reason of possible loss of precision.

-   -   Target t=( )->longVar;

On the other hand, the following expression, which is in a cast context, may actually be accepted by the compiler 120.

-   -   Object o=(Target)( )->longVar;

FIG. 2 illustrates an exemplary flow chart for supporting compatibility checking in a programming language environment in accordance with an embodiment of the invention. As shown in FIG. 2, at step 201, a compiler in the programming language compiling environment can check whether a target type associated with a lambda expression in the programming language environment is functional. Then, at step 202, the compiler can derive a function descriptor from the target type. Furthermore, the compiler can type-check the expression with the function descriptor associated with the target type. For example, at step 203, the compiler can type-check one or more parameter types with the function descriptor; at step 204, the compiler can type-check a return type by a body of the lambda expression with the function descriptor; and at step 205, the compiler can type-check one or more exception types that can result from an execution of the body of the lambda expression with the function descriptor.

Appendix A provides further information with respect to supporting compatibility checking in a programming language environment and various aspects of the platform described throughout this disclosure. The information in Appendix A is provided for illustrational purposes and should not be construed to limit all of the embodiments of the invention.

Wildcard Parameterized Type

FIG. 3 shows an illustration of checking compatibility for a wildcard parameterized type in a programming language environment, in accordance with various embodiments of the invention. As shown in FIG. 3, an expression, e.g. a lambda expression 301, can be supported in a programming language environment 300.

The lambda expression 301 can include an argument list 311 and an expression body 312. The lambda expression 301 can involve different types, such as one or more parameter types 321, one or more return types 322 from the expression body 312 and one or more exception types 323 from the expression body 312.

As shown in FIG. 3, the lambda expression 301 can be associated with a generic functional interface 304, e.g. a functional interface that includes one or more wildcards, in a programming context 310. Thus, the target type 305 for the lambda expression 301 can be wildcard parameterized, in which case a natural parameterized target type 302 may need to be inferred in order for the compiler 320 to check for compatibility.

The compiler 320 can instantiate one or more wildcards in the wildcard parameterized target type 305 and using type inference to infer a natural parameterized target type 302. Type inference is a process of reasoning about unknown types, which can be used as a sub-process of the method for checking compatibility of the lambda expressions.

In accordance with various embodiments of the invention, at a high level, type inference include reducing compatibility assertions about expressions or types, called constraint formulas, to a set of bounds on and dependencies between inference variables. For example, given inference variable α, it might be determined that the constraint formula String[]<: α[ ] reduces to the lower bound α:>String. As bounds and dependencies are inferred, they are incorporated into a bound set. Ultimately, this bound set is resolved to produce an instantiation for each of the inference variables.

In the example of the Java™ programming language environment, to infer a generic method's applicability, a type inference system asserts that the arguments to the method are compatible with their corresponding formal parameter types, and reduces these constraints to a bound set. The system then checks that there exists a resolution of the resulting bounds. To infer the type of a generic method invocation by a lambda expression, the system re-uses the inferred bound set from method applicability testing, and augments it with the results of asserting the compatibility of the method's return type with its target type. The system can also assert that the exceptions thrown by lambda expression bodies are contained by the throws clauses of their target descriptors.

After reduction, the system resolves the inference variables and uses them as type arguments to determine the invocation type of the method. Type inference is also used to determine a target function descriptor for a lambda expression that is assigned to a wildcard-parameterized functional interface type, and also to determine the validity of an unchecked conversion.

When the return type is an inference variable with only an upper bound, the compiler 320 can infer type arguments from the wildcard bounds, the functional interface parameters' bounds, and the explicit lambda parameter types, if available. A more sophistical approach is to further include constraints on the return types and exception types, in addition to inferring type arguments.

Additionally, other strategies can be used for deriving a target descriptor 303 from a wildcard-parameterized functional interface 304. For example, the compiler 320 can perform capture on the functional interface type 304. This approach may not be helpful for a lambda expression 301, since it is difficult to explicitly mention a capture variable in a lambda expression's parameter types, and write a return expression that is compatible with a return type involving a fresh capture variable.

Alternatively, the compiler 320 can simply replace the wildcard with its bound. This approach can be helpful when the bound is an in-bounds type argument. Furthermore, this approach may need to account for the possibility that the user had in mind a different parameterization.

Appendix B provide further information with respect to deriving a target descriptor from a wildcard-parameterized functional interface in a programming language environment and various aspects of the platform described throughout this disclosure. The information in Appendix B is provided for illustrational purposes and should not be construed to limit all of the embodiments of the invention.

The present invention may be conveniently implemented using one or more conventional general purpose or specialized digital computer, computing device, machine, or microprocessor, including one or more processors, memory and/or computer readable storage media programmed according to the teachings of the present disclosure. Appropriate software coding can readily be prepared by skilled programmers based on the teachings of the present disclosure, as will be apparent to those skilled in the software art.

In some embodiments, the present invention includes a computer program product which is a storage medium or computer readable medium (media) having instructions stored thereon/in which can be used to program a computer to perform any of the processes of the present invention. The storage medium can include, but is not limited to, any type of disk including floppy disks, optical discs, DVD, CD-ROMs, microdrive, and magneto-optical disks, ROMs, RAMs, EPROMs, EEPROMs, DRAMs, VRAMs, flash memory devices, magnetic or optical cards, nanosystems (including molecular memory ICs), or any type of media or device suitable for storing instructions and/or data

The foregoing description of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations will be apparent to the practitioner skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications that are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.

Appendix A

The following illustrates supporting compatibility checking in a Java™ programming language environment, in accordance with various embodiments of the invention. It will be apparent to those skilled in the art that other types of programming language environments can be used without limitation.

A lambda expression has a type if it is compatible with its target type.

A lambda expression is compatible in context c with type T if i) a type for the expression can be derived from T, and ii) the expression is congruent in context c with the function descriptor of this derived type.

The expression's type can be derived from the target type as follows:

-   -   If the target type is a wildcard-parameterized functional         interface type, a natural parameterized type is inferred. If a         natural parameterization can be found, this is the result.     -   If the target type is a natural functional interface type, the         result is the target type itself.     -   Otherwise, the result is undefined.

A lambda expression is considered as congruent in a context c with a function descriptor if the following are true:

-   -   The number of lambda parameters is the same as the number of         parameter types of the descriptor.     -   If the lambda parameters have explicitly-declared types, these         types are the same as, or compatible with, the parameter types         of the descriptor.     -   If the lambda body is an expression, then, where the lambda         parameters are assumed to have the same types as, or compatible         types with, the descriptor parameter types, one of the following         is true:     -   i. The descriptor's return type is void, and the lambda body is         a method invocation expression with a void result,         -   ii. The descriptor's return type is a (non-void) type R, and             the lambda body is compatible with R both in an assignment             context and in context c.     -   If the lambda body is a block, then one of the following is         true:         -   i. The descriptor's return type is void, and the lambda body             is void-compatible.         -   ii. The descriptor's return type is a (non-void) type R, the             lambda body is value-compatible, and, where the lambda             parameters are assumed to have the same types as, or             compatible types with, the descriptor parameter types, each             result expression is compatible with R both in an assignment             context and in context c.

A lambda expression may be illegal even if it has a type. For example, it may be compatible with its target type but contain type errors in its body. In addition, where T′ is the type of the lambda expression:

-   -   For each checked exception that can result from execution of the         body of the lambda expression, a compile-time error occurs         unless that exception type or a supertype of that exception type         is mentioned in the throws clause of the descriptor of T′.     -   It is a compile-time error if any class or interface mentioned         by either T′ or the descriptor of T′ is not accessible from the         class in which the lambda expression appears.

Appendix B

The following illustrates deriving a target descriptor from a wildcard-parameterized functional interface in a Java™ programming language environment, in accordance with various embodiments of the invention. It will be apparent to those skilled in the art that other types of programming language environments can be used without limitation.

If the target type of a lambda expression (or) a method reference is a wildcard-parameterized functional interface type I, a natural parameterization of this interface can be inferred in order to test compatibility. This occurs as follows:

-   -   Where P₁, . . . , P_(p) (p≥1) are the type parameters of the         target functional interface, let α₁, . . . , α_(p) be inference         variables, and let θ be the substitution [P₁:=α₁, . . . ,         P_(p):=α_(p)].     -   An initial bound set B is constructed from the declared bounds         of P₁, . . . , P_(p).     -   Let S₁, . . . , S_(p) be the type arguments of the target type.         These contribute to an initial constraint formula set; for         1≤i≤p:         -   i. If S_(i) is a type, the initial constraint formula set             includes S_(i)=α_(i).         -   ii. If S_(i) is a wildcard of the form ? extends S_(i)′, the             initial constraint formula set includes α_(i)<: S_(i)′.         -   iii. If S_(i) is a wildcard of the form ? super S_(i)′, the             initial constraint formula set includes S_(i)′<: α_(i).         -   iv. Otherwise, S_(i) is an unbounded wildcard, and no             constraint formula involving S_(i) is included.     -   If the expression is a lambda expression with declared parameter         types F₁, . . . , F_(n), the set of initial constraint formulas         is further augmented. Let G₁, . . . , G_(k) be the parameter         types of the function descriptor of the target functional         interface. If k=n, then for 1≤i≤n, the constraint formula set         includes F_(i)=ϑG_(i).     -   The constraint formula set, with initial bound set B, is reduced         to a bound set B′.     -   The inference variables in B′ are resolved.     -   If B′ was successfully resolved, resulting in instantiations T₁,         . . . , T_(p)for inference variables α₁, . . . , α_(p), the         resulting type is I<T₁, . . . , T_(p)>. If B′ could not be         successfully resolved, there is no result. 

What is claimed is:
 1. One or more non-transitory machine-readable media storing instructions which, when executed by one or more processors, cause: identifying a first instruction, in a set of code, that associates a particular expression with a first target type in a first context, wherein the particular expression indicates a function and the particular expression is not a member of any class defined by the set of code; performing, during a compilation of the set of code, a first compatibility check on the particular expression and the first target type; determining, during the compilation of the set of code, that the particular expression is compatible with the first target type; identifying a second instruction, in the set of code, that associates the particular expression with a second target type in a second context, wherein the particular expression indicates the function and the particular expression is not a member of any class defined by the set of code, and wherein the first target type and the second target type are different; performing, during the compilation of the set of code, a second compatibility check on the particular expression and the second target type; determining, during the compilation of the set of code, that the particular expression is compatible with the second target type.
 2. The one or more non-transitory machine-readable media of claim 1, wherein performing, the first compatibility check on the particular expression and the first target type comprises: determining that the first target type is a functional interface including a single abstract method; determining a function descriptor associated with the first target type based on at least one of: a method type of the single abstract method, a parameter type of the single abstract method, a return type of the single abstract method, and an exception type of the single abstract method; type-checking the particular expression with the function descriptor associated with the first target type to determine compatibility between the particular expression and the first target type.
 3. The one or more non-transitory machine-readable media of claim 2, wherein: the particular expression specifies a parameter; type-checking the particular expression with the function descriptor associated with the first target type to determine compatibility between the particular expression and the first target type comprises: type-checking the parameter specified by the particular expression with the parameter type of the single abstract method associated with the first target type.
 4. The one or more non-transitory machine-readable media of claim 2, wherein: type-checking the particular expression with the function descriptor associated with the first target type to determine compatibility between the particular expression and the first target type comprises: type-checking a particular return type associated with the function indicated by the particular expression with the return type of the single abstract method associated with the first target type.
 5. The one or more non-transitory machine-readable media of claim 2, wherein: type-checking the particular expression with the function descriptor associated with the first target type to determine compatibility between the particular expression and the first target type comprises: type-checking a particular exception type associated with the function indicated by the particular expression with the exception type of the single abstract method associated with the first target type.
 6. The one or more non-transitory machine-readable media of claim 1, wherein: the first instruction assigns the particular expression to an object of the first target type.
 7. The one or more non-transitory machine-readable media of claim 1, wherein: the first instruction specifies the particular expression as a value for a parameter inputted to a particular method; and a parameter type of the parameter inputted to the particular method is the first target type.
 8. The one or more non-transitory machine-readable media of claim 1, wherein: the first instruction casts the particular expression as the first target type.
 9. The one or more non-transitory machine-readable media of claim 1, wherein: the first context comprises an assignment context; and the second context comprises at least one of: an invocation context and a casting context.
 10. The one or more non-transitory machine-readable media of claim 1, wherein performing, the first compatibility check on the particular expression and the first target type comprises: inferring a natural parameterized type from a wildcard-parameterized functional interface type associated with the first target type; and type-checking the particular expression with the natural parameterized type.
 11. A system, comprising: at least one device including a hardware processor; and the system being configured to perform operations comprising: identifying a first instruction, in a set of code, that associates a particular expression with a first target type in a first context, wherein the particular expression indicates a function and the particular expression is not a member of any class defined by the set of code; performing, during a compilation of the set of code, a first compatibility check on the particular expression and the first target type; determining, during the compilation of the set of code, that the particular expression is compatible with the first target type; identifying a second instruction, in the set of code, that associates the particular expression with a second target type in a second context, wherein the particular expression indicates the function and the particular expression is not a member of any class defined by the set of code, and wherein the first target type and the second target type are different; performing, during the compilation of the set of code, a second compatibility check on the particular expression and the second target type; determining, during the compilation of the set of code, that the particular expression is compatible with the second target type.
 12. The system of claim 11, wherein performing, the first compatibility check on the particular expression and the first target type comprises: determining that the first target type is a functional interface including a single abstract method; determining a function descriptor associated with the first target type based on at least one of: a method type of the single abstract method, a parameter type of the single abstract method, a return type of the single abstract method, and an exception type of the single abstract method; type-checking the particular expression with the function descriptor associated with the first target type to determine compatibility between the particular expression and the first target type.
 13. The system of claim 12, wherein: the particular expression specifies a parameter; type-checking the particular expression with the function descriptor associated with the first target type to determine compatibility between the particular expression and the first target type comprises: type-checking the parameter specified by the particular expression with the parameter type of the single abstract method associated with the first target type.
 14. The system of claim 12, wherein: type-checking the particular expression with the function descriptor associated with the first target type to determine compatibility between the particular expression and the first target type comprises: type-checking a particular return type associated with the function indicated by the particular expression with the return type of the single abstract method associated with the first target type.
 15. The system of claim 12, wherein: type-checking the particular expression with the function descriptor associated with the first target type to determine compatibility between the particular expression and the first target type comprises: type-checking a particular exception type associated with the function indicated by the particular expression with the exception type of the single abstract method associated with the first target type.
 16. The system of claim 11, wherein: the first instruction assigns the particular expression to an object of the first target type.
 17. The system of claim 11, wherein: the first instruction specifies the particular expression as a value for a parameter inputted to a particular method; and a parameter type of the parameter inputted to the particular method is the first target type.
 18. The system of claim 11, wherein: the first instruction casts the particular expression as the first target type.
 19. The system of claim 11, wherein: the first context comprises an assignment context; and the second context comprises at least one of: an invocation context and a casting context.
 20. A method, comprising: identifying a first instruction, in a set of code, that associates a particular expression with a first target type in a first context, wherein the particular expression indicates a function and the particular expression is not a member of any class defined by the set of code; performing, during a compilation of the set of code, a first compatibility check on the particular expression and the first target type; determining, during the compilation of the set of code, that the particular expression is compatible with the first target type; identifying a second instruction, in the set of code, that associates the particular expression with a second target type in a second context, wherein the particular expression indicates the function and the particular expression is not a member of any class defined by the set of code, and wherein the first target type and the second target type are different; performing, during the compilation of the set of code, a second compatibility check on the particular expression and the second target type; determining, during the compilation of the set of code, that the particular expression is compatible with the second target type; wherein the method is performed by at least one device including a hardware processor. 